250 PAPERS FROM THE DEPARTMENT OF MARINE BIOLOGY. 



The count was based on the differentiation of the following groups of 

 organisms: 



Madreporarian corals. Echinoids. Alcyonarian corals. 



Mollusks. Worm-tubes. Bryozoa. 



Calcareous algae. Crustacea. Millepores. 

 Foraminifera. 



Wherever possible, further subdivisions, almost entirely by external 



characters, were made as follows: 



Mollusks: Gastropods, Pelecypods, Scaphopods. 



Foraminifera: Tinoporus baculatus, Amphistegina lessoni, Orbitohtes. 



Crustacea: Balanus or other barnacles (cirripeds), Malacostraca (crabs, lobsters, 



etc.), Ostracods. 

 Alcyonaria: Spicules, solid skeleton. 

 Echinoids: Spines, plates. 



The recognition of the microscopic characters was based on a preliminary 

 study of known material from the different groups, in thin section and more 

 especially as crushed fragments. It is in the study of known material that 

 the work needs particularly to be enlarged, for while the groups differentiated 

 do undoubtedly have certain persistent characters, yet one constantly learns, 

 in studying new, known material in a group, that the differences between 

 members of a group are often more apparent than the resemblances; conse- 

 quently, grains are occasionally encountered in the unknown material, all the 

 most individual characters of which are unfamiliar, or characters are com- 

 bined that seem to ally them to different groups— groups perhaps as remote 

 from each other as algae and crustaceans. The remedy for this confusion is, 

 of course, to study a greater variety of known material under each group; 

 in fact, the most satisfactory procedure would be to collect, at the same time 

 that the sample of sand is collected, a specimen of the skeleton of every species 

 inhabiting the neighborhood of the sample. Then, too, a more detailed and 

 careful study of the materials should be made than the time available in 

 preparation for the study here presented permitted. 



In spite of the difficulties that many of the grains offered, it was deemed 

 best to assign each of them to some form, since a majority of the doubtful 

 grains will probably be correctly identified and the right result consequently 

 more nearly approached than by leaving them unidentified and thus with no 

 effect on the result. Accordingly, only two or three grains, all of one kind 

 and resembling nothing that was known to me, were left undetermined. 



In order to enable the reader to estimate the probability of accuracy in 

 the results here presented, it will be well to indicate in a general way the ease 

 or difficulty with which the groups were differentiated. One of the most 

 important differentiations is that between corals and algae, and this, for- 

 tunately, is one of the easiest and surest. But within each of these groups are 

 two subgroups of very distinct chemical composition. The corals contain 

 the madreporarians (which are nearly pure lime) and the alcyonarians, which 



